1. Field of the Invention
This invention relates to the field of power converters. Particularly, this invention relates to the field of AC to DC converters with Power Factor Correction (PFC).
2. Description of the Related Art
AC/DC converters need power factor correction in order to fulfill international standards of low input harmonic current content. A front-end boost PFC converter is one way to obtain good input harmonic current to meet these international standards. Generally, another DC/DC converter is cascaded from the front-end boost PFC converter to provide a steady output voltage.
FIG. 1 shows a typical configuration of an AC-DC converter with power factor correction. Rectified AC is fed to input terminals of a boost converter 4 at nodes 0 and 1. The boost converter 4 includes an inductor L1 10, MOSFET switch M1 12, diode D1 14 and capacitor C1 16. A series of Pulse Width Modulated (PWM) voltage pulses are fed to the gate terminal G1 of the MOSFET switch 12. The pulse width of the voltage pulses are programmed to make the input current follow the shape of the input sinusoidal voltage and build up a voltage across capacitor 16. A DC/DC converter 20 converts the voltage across capacitor 16 to a regulated DC voltage across output nodes 5 and 6.
A problem in boost converters is the reverse current of the diode 14 when the switch 12 turns on. When the switch 12 turns on, it draws reverse recovery current through the diode 14 and turns the switch 12 off abruptly to block the reverse voltage equal to the output voltage of the boost PFC converter 4. The output voltage is always higher than the peak of the rectified AC and very often is close to 400V. This high output voltage causes a large amount of switching loss when the diode 14 is turned off. This switching loss increases with frequency. However, high switching frequency is often required to reduce the size and weight of the passive components. Thus PFC boost converter 4 generally are lossy circuits due to the high switching frequencies of the circuit. In fact, the switching loss is associated with every switch in the boost converter 4 and every switch in the DC/DC converter 20.
Previous work uses various techniques to reduce switching losses. In U.S. Pat. No. 5,313,382, Farrington discloses a boost converter with an auxiliary switch and a resonant network to achieve reduced voltage stress at a main power switch during turn on. The boost converter also enables a soft turn off of the boost rectifier. The auxiliary switch of the boost converter is turned on without reduced voltage condition, but it has a zero current condition. In U.S. Pat. No. 5,633,579, Kim discloses a boost converter with a stress energy reproducing snubber circuit in order to reduce the stress energy of the boost rectifier during turn off. The snubber circuit reduces the voltage stress on a main switch of the boost converter during turn on. In U.S. Pat. No. 5,748,457, Poon discloses a DC/DC converter which reduces voltage stress by means of zero voltage switching, but it has no boosting and power factor correction effect.
In addition to soft switching, another problem with PFC converters is control of the switching. Some prior art techniques attempt to integrate the PFC converter and the DC/DC converter. Most of these prior art techniques include converters with fewer degrees of freedom which results in restrictions to operate the converters in certain modes, such as the discontinuous mode. These restrictions prevent maximized utilization of all the components.
A power converter includes a boost converter, a DC/DC converter and a coupler chain. The boost converter receives a rectified AC input and generates a high voltage output. The boost converter includes a zero-voltage-switching transistor that is operable to adjust the power factor of the rectified AC input to generate the high voltage output. The DC/DC converter is coupled to the high voltage output of the boost converter and is configured to generate a regulated output voltage. The DC/DC converter includes a complementary pair of zero-voltage-switching transistors that are operable to regulate the high voltage output of the boost converter to generate the regulated output voltage. The coupler chain is coupled between a current-carrying terminal of the zero-voltage-switching transistor of the boost converter and current-carrying terminals of the complementary pair of zero-voltage-switching transistors in the DC/DC converter The coupler chain is operable to reduce the voltage across the current-carrying terminals of at least one of the zero-voltage-switching-transistors.